A tundish carriage for a double-strand continuous caster can have a pair of parallel rails upon which a frame is mounted which receives a trough having casting tubes adapted to communicate with respective continuous casting molds and which can be raised and lowered and can be displaced transversely to the casting direction. The casting molds can be comparatively small parallel-wall or conical-wall molds and the frame is usually comprised of two longitudinal beams and transverse beams connecting the longitudinal beams. The vertices of the substantially rectangular frame are formed with bolsters, each of which can have a wheel riding on a respective rail and a lifter for raising and lowering the trough and provided with an attachment point or support point for the trough on the frame.
In conventional tundish carriages of this type, the trough itself is supported at the bolsters and hence at each of the vertices of the generally rectangular frame symmetrically with respect to the longitudinal axis or a longitudinal median plane through the trough and the frame. The positioning of the casting tubes in molds for the continuous casting of billets must be relatively exact since there is little space to allow for variability in the locating of the casting tubes in such small molds. Hence significant relative movement of the casting tube and the thin-billet parallel-wall or conical-wall molds cannot be tolerated primarily because of the relatively small size of such molds. This will be understood when it is recognized that the spacing of the broad-side copper plates of such molds from one another is for example about 35 mm. As a consequence utilizing the conventional tundish carriage with a rectangular frame and the casting tubes rigidly fixed with respect to one another and the casting frame and other parts of the carriage, significant relative movement could not be provided for a centering process of each casting tube with respect to its mold.
The tundish between the ladle and the casting mold, however, plays a very important role with respect to the quality of the product made. The quality of the continuously cast steel can deteriorate rapidly if undesired elements, like oxygen and nitrogen, are incorporated in the molten metal and hence the uncontrolled incorporation of such elements must be avoided as much as possible. The size and shape of the trough must be chosen so that advantageous rheological conditions are maintained to thereby prevent the entrainment of impurities or to exclude impurities which may be present in the melt from flowing into the mold. Furthermore, from a structural point of view it is important that the tundish be capable of withstanding the stresses of crane transport and melt loading with a minimum of elastic deformation so that there is little if any damage to refractory materials. Finally, residual shape changes resulting from thermal loads, for example, preheating, are not permissible.
Tundishes for two-strand casters have undergone a significant development over the years and while only about ten years ago the average capacity of such a tundish was 15 to 20 tons, today the tundish is required to have a capacity of some 50 to 70 tons. Indeed, while in the past the depth of the molten metal in the tundish was limited to 0.6 to 0.8 meters, currently depths in excess of one meter and lately as much as 1.3 meters are used. As a result, the residence time of the steel melt in the tundish has increased and the flow velocities of the melt have been reduced so that the separation out of impurities at the covering powder is greatly improved.
Tundishes for double-strand molds are generally satisfactory in these latter respects as they are presently constructed. Slider valves or stopper devices have been used to control the outflow from tundishes into continuous casting molds and it has been found that slider valves have a drawback in that air can be sucked into the molten stream through the slider gap. Hence the steel melt admitted through a slide valve may have a greater degree of contamination than a steel melt passing through a casting tube provided with a stopper arrangement.
Japanese open application 61-119358 (application 59-238244) describes a tundish trough for a two-strand continuous caster for producing billet strands. The relatively wide tundish trough is subdivided transversely to the longitudinal direction into two trough parts by a longitudinal partition. One of these parts serves to receive the steel melt from the ladle while the other feeds the casting tubes which are aligned with the two continuous casting molds. The partition is provided close to its bottom with a throughgoing opening that communicates between the two compartments separated by the partition and which thus allows the melt to flow from the receiving compartment to the discharge compartment. Impurities are held back by the partition in the receiving compartment and the product quality is enhanced.
Another problem with respect to earlier tundish arrangements for continuous casting is the difficulty of ascertaining the exact position of the casting or pouring tube with respect to the mold. In most instances the positioning of the pouring tube with respect to the molds could not be visually observed, or if visually observable was at least partly obscured by the tundish carriage structure.